The rejection of bone marrow grafts in mice and other species is mediated largely by natural killer cells. Although the rejection is highly specific, there is a major exception to the rules of transplantation genetics, i.e., F1 hybrid mice (or rats) can reject parental-strain marrow grafts (hybrid resistance). NK cells lack T or B cell receptor genes that rearrange during development. Rather, NK cells express receptors for self and other class I antigens; for the most part, these receptors receive negative signals not to kill target cells. The major receptor gene family is Ly49, on chromosome 6 near the NKR-P1 family of NK receptor genes. Both Ly49 and NKR-P1 molecules are type II integral membrane proteins that exist as homodimers. An immunomodulatory tyrosine-based inhibitory motif (ITIM) located in the cytoplasmic domain is responsible for the negative signaling. Two members of the family, Ly49D and H, lack the ITIM; Ly49D receives activation signals. Co-expression of Ly49D and a negative signaling Ly49 receptor should not kill because the negative signal dominates. The goal of this proposal is to determine how these NK cell receptors regulate rejection of marrow grafts. Aim 1: Determine the role of Ly49C and Ly49I NK cell receptors in rejection of incompatible marrow grafts. Two hypotheses to test: 1) the allelic forms of ITIM+ Ly49C and Ly49I receptors and class I antigens are both critical for the ability of NK cells of various strains to recognize target cells; this ability is also regulated by selective use of these genes; 2) expression of Ly49I gene of B6 mice as a transgene can alter the ability of mice to reject marrow grafts. Aim 2: Determine the mechanism by which the ITIM- Ly49D subsets of NK cells reject marrow grafts differing by particular class I antigens. Two hypotheses to test: 1) the allelic form of Ly49D will regulate the ability of NK cells to reject class I allogeneic marrow grafts. Transfer of the Ly49D gene of B6 to FVB mice that are "poor rejectors" will endow them with the ability to reject such marrow grafts; 2) Ly49D NK cells develop "tolerance" to self class I antigens during development by co-expressing ITIM+ Ly49 receptors that also recognize the self antigen. Aim 3: Analyze the mechanisms by which Ly49- NK cells fail to reject allogeneic marrow grafts. Two hypotheses to test: 1) Ly49- NK cells receive negative signals from most class I Ags on marrow cells; 2) non-Ly49 receptors of NK cells, e.g., NKR-P1B (10A7), that are ITIM+ are lacking on Ly49+ cells except for those that do not recognize "self" class I.